Field of the Invention
The present invention relates to compact low-power consumption type magnetic sensors having high sensitivity and high response speed. More particularly, the present invention relates to high-performance micro magnetic sensors such as magnetic recording heads for computers and information devices; magnetic sensor heads for rotary encoders; and magnetic sensor heads that are used in factory automation so as to carry out measurement for physical distribution, nondestructive magnetic inspection, etc.
Computers and information devices are required to have drastically increased storage capacity so as to cope with multimedia techniques. Therefore, in the field of magnetic disk memory, there is a rapid movement to replace magnetic heads with magneto-resistance (MR) elements. Another movement is the utilization of a giant magneto-resistance effect (GMR) to obtain a highly sensitive magnetic head, with expectation of realizing a hard disk unit having a storage density of 10 gigabits/square inch by about 2010. In order to obtain a hard disk unit having a storage density greater than 10 gigabits/square inch, a magnetic head having higher sensitivity is needed, and a magneto-impedance (MI) element is a promising candidate for such a magnetic head.
Meanwhile, in the field of industrial instruments/control as well as in the field of measurement related to medicine, the environment, disaster prevention, nondestructive inspection, etc., there is arising a strong demand to detect magnetic fields having a strength of one microgauss to a few gausses within a frequency range of 0 to 10 MHz.
In the European Union, there was introduced in January, 1996 a regulation regarding generation of electromagnetic noise from electronic equipment and exposure thereto, and since then a demand for sensors capable of detecting very weak electromagnetic waves has become stronger.
In order to detect a weak magnetic field or wave within the above-described frequency range, a micro-dimensioned head of a Hall element or MR element can be used. However, the sensitivity of such a head is insufficient. When a flux gate sensor is used for such purpose, sufficient response speed cannot be obtained. The flux gate sensor also has a problem that its sensitivity becomes quite insufficient when formed in a micro-sized head.
To overcome the above-described drawbacks, the inventors of the present invention have proposed an improved magneto-impedance (MI) element (see Japanese Patent Application Laid-Open (kokai) No. 7-181239). A micro-dimensioned head utilizing the improved MI element has a resolution of one microgauss and a high responsiveness of 1 MHz, thereby covering a large portion of the required range for magnetic field detection. The power consumption of the micro-dimensioned head is as low as about 30-50 mW.
Since the above-described MI micro sensor is a high performance sensor having a resolution of one microgauss and a high responsiveness of 1 MHz, it can solve many of the technical problems related to new types of magnetic detection required in many industrial fields.
Accordingly, MR elements, Hall elements, and flux gate sensors, which have been widely used, are expected to be replaced with MI elements in the near future so as to realize more advanced systems.
However, there must be solved many problems related to the replacement of currently used elements with MI elements.
One such problem is difficulty in designing a sensor circuit and stems from the necessity of high-frequency technology. Another problem is power consumption in the case where many MI elements are used.
The former problem has been basically solved through use of a high-frequency oscillation type magnetic head in which the magnetic head is driven by means of a Colpitts oscillation circuit or a multivibrator circuit.
However, it is desired to further decrease the number of components of such an electronic circuit.
The latter problem is minor because the power consumption of a single MI element available at the present is about 30-50 mW, and this is sufficiently low for the element itself. However, when many MI elements are used in, for example, an information device, the maximum allowable power consumption of the MI element is considered to be further decreased (for example, to be less than 1 mW).
In order to solve these problems, the use of field effect transistors (FETs, MOSFETs) in place of bipolar transistors (BPTS) has been considered. However, the above-described problems cannot be solved through simple replacement of BPTs with FETs or MOSFETs, because in general an excitation current of not less than a few milliamperes must be caused to flow through a magnetic body in order to sufficiently utilize the magnetic effect of the magnetic material.